1. Field of the Invention
The present invention relates to a vehicular camera that is mounted on a vehicle, shoots the surroundings, and, in particular, is used for detecting a desired object from a resulting video signal with such a means as image processing.
2. Description of the Related Art
In recent years, vehicular cameras that are mounted on an automobile (a CCD camera (i.e., a camera using a solid-state image pickup device) is mounted on the automobile) and that recognize white lines on the road on which the automobile is running, detect an obstacle, and perform like operations have been developed as apparatus for securing safety driving of automobiles.
For example, a camera disclosed in JP-A-9-83879 (hereinafter referred to as patent document 1) is known as a conventional vehicular camera, in particular, a camera that is premised on image processing and exhibits a pronounced effect when using an optical filter.
The camera of patent document 1 is such that a polarizing filter and an infrared filter are integrated with each other and incorporated in a camera, whereby inclusion of undesired light coming from the windshield can be prevented and reflection light from a road surface can be eliminated without causing deterioration of the polarizing filter by direct sunlight or occurrence of a ghost image. As a result, white lines on the road on which the automobile is running, nearby vehicles, etc. can be shot clearly.
For example, when a scene in front of the automobile is shot with a video camera, there may occur a case that lane boundary lines and vehicles ahead are buried in reflection light from the road surface that is caused by shooting against the sun or the road surface being wet, to disable recognition by image processing.
The vehicular camera of patent document 1 eliminates only a horizontally polarized reflection light component, and hence can shoot white lines and vehicles ahead clearly even under bad conditions as described above and facilitates later image processing as post-processing.
Incidentally, in cameras such as the above-described vehicular cameras that are premised on image processing, it is very important to obtain camera performance that conforms to the object or algorithm of the image processing. For example, in image processing for recognizing traffic lane boundary lines, it is a general procedure to perform the image processing paying attention to the fact that the luminance of the lane boundary lines are higher than that of the road surface. Cameras are required to reproduce the luminance relationship between the road surface and white lines faithfully at proper contrast.
Roads in which white lines are formed on an asphalt road are free of problems because the luminance of the white lines are sufficiently higher than that of the road surface. However, problems may occur in roads in which yellow lines are formed on an asphalt road or white lines are formed on a concrete road, because the difference in luminance between the road surface and the lane boundary lines is small and the contrast of a video signal is low. This adversely affects the cognitive performance of the image recognition such as the recognition rate, the reliability of recognition, and the stability.
Where yellow lines are formed on a concrete road, the luminance of the yellow lines is lower that that of the road surface. This results in a problem that the lane boundary lines cannot be recognized if the relative luminance relationship between the road surface and the lane boundary lines is reproduced faithfully.
Polarizing filters that are used in conventional vehicular cameras as exemplified by the one disclosed in patent document 1 cannot solve the above problems because they do not change the relative luminance relationship except for a case of handling polarized light.